The present invention relates to methods for manufacture of titanium dioxide pigments that are encapsulated with a dense silica layer and have improved optical properties.
Titanium dioxide (xe2x80x9cTiO2xe2x80x9d) by virtue of its high refractive index and low reactivity has become the premier white pigment of commerce. However, titanium dioxide is also a photocatalyst that can contribute to the destruction of polymeric binders in polymers in which it is incorporated. Manufacturers of titanium dioxide pigments (also called xe2x80x9ctitaniaxe2x80x9d pigments) have developed a number of surface treatments whereby inorganic and organic compounds are added to the raw titania base to both reduce the photocatalytic activity and improve the degree to which the pigment is dispersed. Typical inorganic surface treatments are usually precipitated from an aqueous solution as oxides or hydrated oxides of elements including titanium, aluminum, phosphorous, zirconium, tin and silicon, or mixtures thereof.
Of the options available, so far only silica treatments have been able to deliver maximum durability on a commercial level. More particularly, the silica is required to be precipitated in a dense form to optimize durability. Unfortunately, dense silica treatments have not been able to match the opacity obtainable with other inorganic surface treatments such as titania and zirconia. Since the primary role of titanium dioxide has been as an opacifier, the market has been split into two general classes of xe2x80x9cdurablexe2x80x9d and xe2x80x9cuniversalxe2x80x9d or high opacity grades. Whilst there has been movement towards higher opacity dense silica treated grades over the last decade, opacity is usually improved by reducing the silica level from those typically established on the benchmark xe2x80x9csuperdurablexe2x80x9d grades of titania.
Dense silica treatments of oxides, including titanium dioxide, are described in the prior art. Methods for improving dispersability or optics of dense silica treated pigment are also known, including for example, by the addition of a topcoat of alumina or by using combinations of dense and porous silica.
The use of a milling step after dense silica treatment and washing, but before drying, has been found to produce pigment having a good particle size distribution. The use of steric stabilizers such as alkali metal salts of organic fatty acids for silica treatments has been used to prepare non-agglomerated water insoluble particles having a dense amorphous silica coating. The use of ultrasonic vibrations for at least part of the dense silica precipitation has also been used to produce satisfactory coatings of dense silica on inorganic particles at a moderate temperature and at an acceptable rate. The use of monomeric precursors, such as tetraethylorthosilicate, to obtain a substantially uniform and homogenous dense silica coating has also been triedxe2x80x94with the expectation of reducing treatment levels and producing higher gloss.
While the above-mentioned processes attempt to overcome the compromise between gloss, optics and durability, none have developed a process which truly overcomes the problem and provides a high durable, dense silica coated pigment with good optics and gloss properties.
The invention relates to an improved titanium dioxide pigment that incorporate a dense silica coating and its process for manufacture. The pigment is preferably produced from a titania base from the chloride process in which the manufacturing process comprises the following steps in the order stated: (a) partial or substantial removal of salt from the oxidizer discharge; (b) addition of a dispersant; (c) milling to produce a highly dispersed, highly deagglomerated slurry, followed by dense silica surface treatment.
Pigments prepared using the inventive process exhibit optics equivalent to high gloss, high opacity pigments and maintain the durability of the more conventional dense silica treated pigments.